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Science 3210 001 : Introduction to Astronomy
Lecture 8 : Star Formation
Robert Fisher
Items
Reading/Homework set 7 has been posted to the website.
Solution sets 3-4 have been posted to the website.
March 23 (next week!) of spring break -- no class!
Review of Lecture Two Weeks Ago
The Outer Planets
Review of Last Week
Extrasolar planets
51b Peg
HD209458b
Today -- Star Formation
Star Formation Interstellar Chemistry, or Where to Stop For Alcohol on Your Next
Interstellar Road Trip. Formation of Protostars
Stellar Binaries
Stellar Clusters
Characterizing Stellar Properties
Interstellar Medium
The interstellar medium plays a crucial role as the ultimate reservoir of material from which stars form.
The material released from stars over their lifetimes goes back into the interstellar medium to replenish it.
Stars
ISM
Winds, SNe
Interstellar Medium
The space between the stars is not empty, but contains a complex though highly dilute mixture including Gas
Dust
Starlight
Cosmic Rays
Magnetic Fields
Determining the Composition of the Interstellar Medium
How can one go about determining the composition of the interstellar medium, which about one hydrogen atom per cubic centimeter on average -- about a million billion billion times less dense that water?!
Absorption Lines
By measuring the absorption spectra along lines of sight to distant stars, astronomers can infer the amount of material in the ISM to those stars.
Piecing together the overall 3D structure of the interstellar medium is then like assembling a massively complex jigsaw puzzle.
Interstellar Clouds
Many decades of painstaking observations reveal a fascinatingly complex structure to the ISM.
Most of the gas is concentrated in “clouds” of various types, and ranges from tens to millions of degrees in temperature.
Hydrogen clouds -- an “average” interstellar cloud containing mostly atomic hydrogen at hundreds to thousands of degrees, with roughly a few atoms per cubic centimeter.
Giant Molecular clouds -- very dense and cold (by ISM standards) -- hundreds of thousands of molecules per cubic centimeter, tens of degrees. Location of all known star formation.
Coronal phase -- very hot (millions of degrees), heated by powerful blasts from supernovae. Diffuse, not concentrated in clouds.
Gas Pressure
A gaseous system exerts a force on its surroundings.
On a small scale, this is due to the random thermal collisions of atoms and molecules, like balls in a bingo machine.
Pressure
When a gaseous atom or molecule collides with walls of the container, it bounces, which results in a change in its velocity.
This change in velocity is an acceleration, which is also a force by Newton’s second law.
Pressure is defined to be force, per unit area.
Force
Gas Laws
From the late 17th through early 19th centuries, scientists studied how gas pressure behaved, and found it followed simple mathematical laws.
The hotter the gas, the greater the pressure.
The more confined (and more dense) the gas, the greater the pressure.
The more gas, the greater the greater the pressure.
Why Stars Form -- A Competition Between Gravity and Pressure
All phases of stellar evolution -- from its formation through the end of its life -- are determined by the competition between gravity and pressure.
Gravity binds a star (or cloud of gas) together. The more massive the star, the smaller its radius, the stronger the effect of gravity.
Gas pressure pushes outward and acts against gravity. The hotter the gas, the greater the pressure.
The Competition Between Gravity and Pressure
If gas pressure exceeds gravity, the gas is blown outwards.
As the gas expands, it cools.
Gravity Gas Pressure
The Competition Between Gravity and Pressure
Conversely, if gravity exceeds gas pressure, the region collapses.
As the gas contracts, it is heated.
Gravity Gas Pressure
The Competition Between Gravity and Pressure
If gravity balances gas pressure, the star remains in hydrostatic equilibrium.
Gravity Gas Pressure
Question
If a cloud of gas collapses and compresses, does it cool down or heat up?
Gravity Gas Pressure
Question
What might stop the collapse?
Gravity Gas Pressure
Question
If the cloud cooled by radiating away energy, so that its temperature remained constant, would it stop collapsing?
Gravity Gas Pressure
Dust from Space -- Brownlee Particles
Measuring the properties of interstellar dust particles may seem like an impossible task.
However, it is possible to capture cometary dust in high-flying aircraft. It is believed these dust grains are quite similar to those in the interstellar medium.
Formation of Molecules in Space
When radio and millimeter technology first became available in the 1960s, theorists predicted that it would be impossible for atoms to combine to form molecules in the vastness of space.
Molecules in Space
Instead, astronomers found that molecules were very common in interstellar space -- particularly CO and NH3.
The most common molecule in interstellar space -- H2 -- doesn’t emit this type of radio emission and so is nearly invisible.
CO Map of Horsehead Nebula (BIMA/M. Pound)
Molecules in Space
As time progressed, astronomers have discovered increasingly complex molecules like formaldehyde (H2CO) and ethyl alcohol (C2H6O) in molecular clouds.
These complex molecules would require not only two atoms colliding and sticking, but many such collisions.
Such a gaseous phase process is indeed impossible at the low densities of interstellar space, and even more so once the disruptive effects of cosmic rays and UV radiation from starlight are taken into account.
How then can molecules form in interstellar space?
Molecule Formation on Dust Grains
It turns out that dust grains, although comprising only about 1% of the mass of a giant molecular cloud by mass, play an essential role in catalyzing the formation of molecules :
1)Individual atoms impact the surface of the grain and adhere to it.
2) Over very long times -- thousands of years -- the atoms bounce around over the surface of the grain due to random thermal motion.
3) Eventually two atoms encounter each other on the surface of the grain and combine to form a molecule, which then leaves the surface after releasing its heat of formation.
Cooling by Dust Grains
Besides playing an essential role in the formation of molecules, dust grains also play a key role in the energy budget of a giant molecular cloud.
Energy budget is key to determining when stars form -- cooling leads to loss of pressure support, which enhances star formation.
The gas is heated by a variety of sources -- starlight, cosmic rays, and gravitational contraction.
At the same time, the gas radiates away light energy in the radio portion of the spectrum, which cools the gas.
Cooling by Dust Grains
At low densities, the radiation from molecules can escape freely, but at higher densities the radiation is absorbed.
At these higher densities, dust grains continue to cool the gas indirectly, through collisions.
An atom or molecule collides with the dust grain, which then radiates away the energy in the infrared.
Infrared Radiation
Cooling by Dust Grains
Eventually the gas becomes so dense that it absorbs the radiated infrared radiation as well.
At this point the gas is “optically thick,” and will heat up.
From the standpoint of star formation, heating the gas is a crucial step towards arresting the collapse of the gas and beginning to form the “first core” which will form a star.
From the standpoint of astronomical observers, the dust provides a sensitive view of even the densest regions of a giant molecular cloud, even when gaseous lines become optically thick.
Other Supporting Forces
In addition to gas pressure, other pressure support from different physical processes can act to help support the star -- these include light pressure, magnetic pressure, and turbulent pressure.
Each of these mechanisms shapes the process of star formation in different ways.
The detailed picture resulting from all of these effects is still being actively researched and so remains at the forefront.
Light Pressure
The light from the most massive stars is so intense that it exert a significant influence over its surroundings, as in this Spitzer image of the Eta Carina nebula.
The Influence of Turbulence
In the 1990s, astrophysicists first began to seriously consider the influence of turbulence on star formation.
Earlier models had assumed that star formation occurred in spherically-shaped giant molecular clouds, even though the majority of stars form in highly-complex clouds that bear no resemblance to a sphere whatsoever.
Turbulence in the Interstellar Medium
The complex structure seen in real giant molecular clouds eluded the explanation of simple spherical models.
Simulation of Star Formation in a Turbulent Gas Cloud
Star Formation
Imagine an idealized model of star formation, from a spherical parent gas cloud (sometimes called a “molecular cloud core”) without turbulence or magnetic fields.
Initially, before the star has formed, the parent gas cloud is a state of hydrostatic balance.
10,000 AU
~ 1 Solar Mass
Star Formation
The loss of pressure support (possibly through cooling) leads to gravitational collapse of the cloud.
As the cloud collapses, the effect of rotation becomes more significant. The cloud flattens into a collapsing, rotating thin disk.
100 AU
Molecular Core
The collapse of the disk is arrested at the center once the gas begins to heat up and can support itself under its own weight.
At this point, the central “core” is entirely molecular in composition, is a few hundreds of degrees at its surface, and has a radius of a few AU (comparable to the orbit of Jupiter).
This object is sometimes called the first, or molecular core. It is still far too cold to ignite nuclear reactions.
100 AU
A Protostar is Born
The first molecular core continues to radiate energy, and must obtain this energy from some source.
Its only reservoir is gravitational energy, so it is forced to contract, which causes it to heat up further.
Eventually the temperature becomes high enough to dissociate molecules inside the first core, which leads to overall collapse.
This process is arrested once more, producing a protostar which begins to ignite nuclear reactions in its core to power it.
Observational Evidence for Young Protostars
Astronomers have seen some nearby young stellar systems which are roughly consistent with this timeline for the formation of young protostars.
Winds, Jets, and Outflows
Once they begin to burn hydrogen, protostars power spectacular outflows moving at speeds of hundreds of thousands of miles per hour back into the interstellar medium.
XZ Tauri
This time-lapse movie for the binary XZ Tauri was taken over three years on the Hubble Space Telescope. The image covers roughly 1000 AU.
The outflow appears to be highly sporadic, but it remains unclear how it is being powered or even which binary member is powering it.
HH 30
Perhaps the most famous outflow system. This set of images taken over several years by Hubble reveal the powerful jet moving at hundreds of thousands of miles per hour is precessing like a top over time, and is also highly episodic.
Binary Stars
Binary Stars
The majority of stars (unlike our sun) exist in bound systems of two stars orbiting about one another.
Artist’s Conception of a Red Giant Orbiting a Black Hole
Binary Stars
Binary stars are significant because they allow the masses, periods, and separations of each star to be accurately determined.
The orbits remain relatively fixed over time, so knowing the amount of angular momentum in the system gives us an additional clue about how the stars formed.
Primary Secondary
Center-of-Mass
Visual Binaries
Some binaries have wide enough orbits that the stellar components can be resolved in a telescopic image.
In these cases, the binary period, orbital separation, and masses can all be determined directly.
Hubble Image of Sirius A/B Visual Binary
Sirius A
Sirius B
Spectroscopic Binaries
In some cases, the two binary stars are close enough that they cannot be resolved in a telescopic image.
The Doppler technique can be used to detect many binaries spectroscopically that could not have been detected visually.
Double-line Spectroscopic Binary
Spectroscopic Binary
Observations of the stellar spectra over time reveal the period of the binary system as well as the separation, and hence the masses.
Eclipsing Binaries
Some binaries, like planetary transits, eclipse one another along our line of sight to them.
Measuring the light curve from the system gives us both stellar radii in addition to the stellar masses and orbital separation.
Alvan Clark (1804 - 1887) and the Discovery of Sirius B
Alvan Clark was the foremost telescopic lens manufacturer of his time.
Manufactured lenses for Naval Observatory (where Pluto’s moon Charon was discovered) and the University of Chicago Yerkes Observatory (which just shut down research very recently).
In 1862, when testing a new 18 inch telescope at the Dearborn observatory at Northwestern University in Evanston, he discovered a companion to Sirius -- Sirius B.
Sirius A/B
Sirius B turns out to be an eclipsing binary, so that its radius can also be determined from the eclipse measurements.
These observations revealed a highly unusual structure -- a mass about that of the sun, and a radius about that of the Earth.
Sirius B became the first-known white dwarf star. How it managed to support itself against gravity would require entirely new physics.
Artist’s Conception of Sirius A/B
Demographics of Young Binaries
Period distribution of binaries informs our understanding of star formation.
Stellar Clusters
Stellar Clusters
Stars rarely form in isolation. Most stars form in giant molecular clouds with enough material to form tens of thousands to hundreds of thousands of stars.
These stellar clusters are gravitationally bound to one another.
Two major types of stellar clusters can be distinguished on the sky.
These two types of clusters are thought to have very different formation mechanisms -- in particular, globulars are known to be ancient, dating to the formation of the galaxy, whereas open clusters are much younger.
Pleaides
The most famous open cluster of stars is the Pleaides cluster.
Pleiades
Pleiades in the X-ray Band
The brightest stars of the Pleiades are actually only the tip of the iceberg -- many more stars are members of the cluster, as is evident in this X-ray image.
Nebra Sky Disk
The Bronze Age Nebra sky disk is one of the oldest known representations of the night sky -- dating from c. 1600 BC Germany.
It is believed that the Pleiades is represented in the upper right of the image.
Globular Clusters
Globular clusters are some of the most magnificent sights in the night sky, containing hundreds of thousands of stars in a relatively compact space of a few tens of thousands of light years in diameter.
The central densities of the cluster become high enough that stellar collisions can occur.
There is some evidence for these stellar collisions in “blue stragglers”.
There is also long-standing speculation, and some evidence that continued stellar collisions may lead to massive black holes of thousands of solar masses at the center of the globular.
The Globular Cluster M80
Galactic Distribution of Globular Clusters
Globular clusters are distributed in a sphere around the galaxy.
Other disk galaxies have been observed to have their own system of globular clusters surrounding them.
Some globulars may pass through the plane of the galactic disk from time to time, stripping away some stars in a “disk shocking”.
Stellar Associations
Open clusters eventually become less dense over time, and form a loosely-packed, unbound stellar association that will eventually break apart.
Membership in the association can only be confirmed by inspecting the motions of the stars on the sky -- their proper motions -- carefully.
Christmas Tree Cluster
One example of an association is the Christmas Tree cluster.
Like all other associations, it is unbound and will eventually move apart.
Stellar Properties
Astronomers have historically characterized stars by their color. From our knowledge of the blackbody radiation emitted by all bodies, we know this stellar color translates directly into surface temperature.
Mnemonics
The spectral classes can be easily remembered by
Oh Be A Fine Girl/Guy Kiss Me
Many other mnemonics have been created -- or make up your own!
Oh Boy, Astronomy Final's Gonna Kill Me
Out Back A Friend Grows Killer Marijuana
Oven Baked Ants Fried Gently Keep Moist
Only Boring Astronomers Find Gratification Knowing Mnemonics
Stellar Magnitudes
Astronomers will often classify the brightness of stars by their magnitude.
The original classification was meant to agree loosely with an ancient system due to Ptolemy -- magnitude 1 stars are among the brightest on the sky, and magnitude 6 stars are among the faintest visible to the naked eye.
Each increment on the magnitude scale represents not a linear shift, but a multiplicative factor of 2.5. A magnitude 2 star Is 2.5 times fainter than a magnitude 1 star, and so on.
Stellar Magnitudes
It is important to realize that these magnitude ratings reflect the apparent brightness of a star. Two stars of the same intrinsic brightness at two different distances will have two different magnitudes.
If one also knows the distance to the star (not always the case!), then one can correct for the distance and obtain an intrinsic magnitude. By convention this is chosen to be a distance of 10 pc, or 32 LY.
Examples of apparent magnitudes Sun -26.3 (intrinsic 4.8) Vega 0 Uranus 5.5 Pluto 13 HST Limit30
Charting Stars -- The HR Diagram
Around the beginning of the 20th century, two astrophysicists -- noticed something fundamental about the properties of stars.
They compared stellar properties by displaying intrinsic stellar luminosity along one axis and temperature along the second axis.
HR Diagram
When displayed in this fashion, definite patterns popped out immediately.
Question
Which star is larger, A or B?
BA
Classification of Stars
This classification system helped identified the major classes of stars :
Main Sequence. This is where all stars begin their lifespans burning hydrogen, and where they spent most of their life. (Example : our sun.)
Giant Branch. After stars deplete their supply of hydrogen they swell up to an enormous radius and begin burning hellium and heavier elements on this branch. (Example : Alderberan.)
Supergiants. Among the brightest stars in the universe, shortly before the end of their lifespan. (Example : Rigel.)
White Dwarfs. Stars similar in mass to our sun will wind up as white dwarfs -- extremely dense, hot stellar remnants. (Example : Sirius B.)
Question
Which star is more evolved, A or B?
A
B
Two Weeks from Today -- Stellar Evolution
